adding documentation for --enable-languages

[Matthias Klose]

To add documentation for this option, I'd like to verify: The option
can be followed by a comma separated list. Valid values are:

c	 - C compiler
c++	 - C++ compiler, libstdc++ and libg++, if found in the tree
	   (correct?)
java	 - Java compiler zlib, (libjava, libgc, libffi, when found in
	   the tree, correct?)
f77	 - f77 and runtime
objc	 - ObjC compiler and runtime
CHILL	 - CHILL compiler and runtime
proto	 - protoize/unprotoize programs
gcov	 - gcov program (why gcov$(exeext) in gcc/Makefile.in?)

c, proto and gcov cannot be disabled.

The default is to build all languages but CHILL.